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Biomedical subjects

S Sideman

Publications and source records attributed to S Sideman.

At least 73 records · Page 4Linked to original sources
PubMed1986

The dynamic twisting of the left ventricle: a computer study.

A mathematical analysis which relates the dynamic twisting motion of the heart around its longitudinal axis to the mechanical function of the left ventricle (LV) is presented. The study thus extends our earlier model which relates the micro-scale sarcomere dynamics, the fibrous structure of the myocardium, and the electrical transmural activation wave to the global LV function. The analysis demonstrates that although the angular twisting motion of the heart moderates the sarcomere length (SL) and the strain rate distributions throughout the myocardium, the global characteristics of the LV function are almost independent of the twisting phenomenon. The endocardial sarcomeres are nevertheless subjected to higher strains and higher (negative) strain rates than the corresponding (positive) epicardial sarcomeres. Utilizing the sarcomere stress length area to predict oxygen demand, it is shown that the twisting motion of the heart produces the metabolic gradient across the LV wall. In spite of the moderating effect of the twist, a larger than normal gradient in oxygen demand is predicted for cases of concentric hypertrophy.

Aortic Valve Stenosis↗
PubMed1986

Source parameters of the left ventricle related to the physiological characteristics of the cardiac muscle.

An attempt is made here to correlate the physiological muscle parameters with the dynamic source parameters of the left ventricle (LV), i.e. the source (isovolumic) pressure Po and the source (internal) resistance, Rs. The internal resistance is described here as a time-dependent parameter, corresponding to the pressure drop (from the theoretical instantaneous isovolumic pressure) associated with the instantaneous ejection flow rate. The source pressure, which relates to the muscle stress and the ventricular volume, is represented by the time-varying elastance concept and a spheroidal model relating the average wall stress to LV pressure. Linear and exponential force-velocity relationships (FVR), expressed in stress-strain rate terms, are compared. Two possible characteristics of the dynamic FVR in the partially active state, based on either a parallel or a fanlike shift of the stress-strain rate curve, are studied by utilizing simple analytical models as well as a computer simulation model. Comparing the calculated results with experimental data indicates that the dynamic FVR shift occurs in a fanlike pattern in which the maximum strain rate remains constant throughout the cycle. This pattern of the FVR shift is consistent with experimental data that show that the internal resistance is linearly related to the instantaneous isovolumic pressure. The analysis also indicates that the difference between the hyperbolic and linear FVR is rather minor, and in spite of some effects on the ejection pattern and the value of Rs, the functional shape has no effect on the global LV characteristics, such as the ejection fraction and stroke volume.

Animals↗
PubMed1986

Augmentation of cardiac output and carotid blood flow by chest and abdomen phased compression cardiopulmonary resuscitation.

Phased compression cardiopulmonary resuscitation, whereby the chest and abdomen are compressed sequentially, is a new approach to the classical cardiopulmonary resuscitation technique, which is based on the compression of the chest alone. Six dogs with cardiac arrest were treated by external chest and abdominal compression using a rigid plexiglas suit lined with flexible perithoracic and periabdominal bladders. Fast inflation and deflation of the two independent bladders, together with forced ventilation of the lung, generated phased pressure pulses. The physiological variables monitored throughout the experiment included central venous, left ventricular, and central arterial pressures, carotid blood flow, cardiac output, and acid base balance. The phased compression technique was performed with phased time lags of 0, 150, 300, 400, 600, 700, and 850 ms between the abdominal and thoracic pressure pulses. A random sequence of the different phased compression modes, each lasting for 3-10 minutes, was applied during the prolonged resuscitation procedure that lasted for up to 70 minutes. By starting the abdominal compression 300-400 ms before the thoracic compression the carotid flow index improved by 77% (from 13% with simultaneous compression to 23% with phased compression) and the cardiac output index increased by 65% (from 7.8% with simultaneous compression to 12.5%). The results provide insight into the chest pump concept and the role of intrathoracic and intra-abdominal pressures in generating improved blood circulation during cardiopulmonary resuscitation, and show the advantages of phased compression over chest compression alone and simultaneous chest and abdominal compression.

Animals↗
PubMed1986

Relating left ventricular dimension to maximum elastance by fiber mechanics.

The dependence of the pressure-volume slope, which defines the maximum elastance (Emax) and the zero pressure-volume intercept (Vd) on the size and dimensions of the left ventricle (LV), is theoretically studied, and a normalizing parameter for Emax is suggested for normal and hypertrophied hearts. The study is based on our earlier model of the mechanics of the LV contraction, which assumes a nested-shell spheroidal shape, Streeter's fiber angle distribution, given stress-length and stress-strain rate functions of the sarcomeres, a radial propagation of the electrical activation front, and a windkessel arterial model. The study shows that Emax is linearly related to the maximum force that the optimal length sarcomeres can develop (sigma o), which is a characteristic measure of the contractility. Emax decreases and Vd increases with an increase in ventricular size, at a constant end-diastolic ratio (h/b)ed, where h is the wall thickness, and b is the semiminor axis of the prolate spheroidal LV. When the reference unstressed volume (V0) is held constant and the wall thickness increases, as in pure concentric hypertrophy, Emax decreases slightly and shifts to the left to a lower Vd value. In pure eccentric hypertrophy, wherein chamber size increases while the wall thickness remains constant, Emax decreases and Vd increases. A good index for myocardial function at constant configuration ratio (h/b)ed is obtained by multiplying Emax with the LV muscle volume (Vm). (h/b)ed is constant (= 0.45) for the normal heart but increases for concentric hypertrophy.(ABSTRACT TRUNCATED AT 250 WORDS)

Biomechanical Phenomena↗
PubMed1986

Left ventricular mechanics related to the local distribution of oxygen demand throughout the wall.

The complex interactions between left ventricular mechanics and the oxygen demand is studied by relating the left ventricular transmural oxygen demand to the myocardial structural and dynamic characteristics. The study utilizes a recent model of left ventricular contraction, which is based on a nested shell spheroidal geometry, a fan-like fibrous structure, the twisting motion of the left ventricle over its long axis, a transmural electrical activation propagation and the basic laws of sarcomere dynamics. The local "axial" stress (in the direction of the fibers) and the instantaneous sarcomere length are used to calculate the spatial distribution of the intramural oxygen demand per beat Vo2(y), where y is the distance from the endocardium. The normalized local sarcomere stress-length area SLAn(y) is related linearly to Vo2(y) by: Vo2(y) = K1 X SLAn(y) + K2, where K1 and K2 are constants. The calculations show a transmural metabolic gradient which is characterized by higher values of Vo2(y) in the endocardial layers than in the epicardial layers. Shorter endocardial sarcomeres and the twisting motion of the left ventricle around the long axis decrease the metabolic gradient across the wall, while a slow transmural electrical propagation wave as well as a wider angle of distribution of the fan-like fiber architecture increases the transmural metabolic gradient. Integration of the local oxygen demand across the left ventricular wall yields global values in agreement with those based on Suga's pressure-volume area approach. The model thus provides a qualitative and quantitative tool to assess the relation of the local and global oxygen demand to the complex left ventricular structure, fiber mechanics, and the dynamics of contraction.

Computers↗
PubMed1986

Enzyme-based hemoperfusion and blood treatment.

Enzyme-based artificial organs are being developed as metabolic assist devices. These are required when normal metabolism is impaired, or when the body is overloaded by undesired metabolites or toxins. The implementations of this approach for treating a genetic disease, and for metabolic support in liver failure are envisaged. The kinetic aspects and mass transfer characteristics of bioreactors for these systems are considered in detail.

Blood↗
PubMed1986

The "jaundiced heart": a possible explanation for postoperative shock in obstructive jaundice.

Patients with obstructive jaundice are susceptible to postoperative shock and kidney failure. The cause of these potentially fatal complications has not been fully clarified. The present study was designed to assess the role of myocardial dysfunction in the hemodynamic disturbance of obstructive jaundice. We studied the effect of isolated cholemia on left ventricular performance in five conscious dogs before and 2 weeks after choledochocaval anastomosis by using measurements of systolic time intervals (STIs) and maximal dp/dt. Mean left ventricular ejection tiem (LVET) decreased after cholemia from 159 +/- 2.8 msec to 139 +/- 2.6 msec (p less than 0.005), while mean preejection period (PEP) and mean PEP/LVET were increased from 41 +/- 8.5 msec to 87 +/- 14 msec (p less than 0.05) and from 0.39 +/- 0.06 to 0.62 +/- 0.1 (p less than 0.01), respectively. During cholemia, STIs were unchanged after intravenous administration of ouabain, whereas in the control period, there was shortening of mean PEP from 71 +/- 8.8 msec to 58 +/- 7.6 msec (p less than 0.05) and of Q-S2 from 257 +/- 12 msec to 235 +/- 14 msec (p less than 0.005) in response to ouabain. Maximal dp/dt decreased after choledochocaval anastomosis from 4543 +/- 593 mm Hg/sec to 3666 +/- 648 mm Hg/sec (p less than 0.025). We conclude that cholemia in the dog is clearly associated with impaired left ventricular performance. The present data also support a previously published in vitro study from our laboratory showing that cholemia blunts the myocardial contractile response to sympathomimetic agents. The cardiodepressor effect of cholemia may explain the increased tendency of patients with obstructive jaundice to postoperative shock and renal failure.

Animals↗
PubMed1985

Serum bilirubin constituents in different experimental models of conjugated hyperbilirubinemia.

The object of the study was to explain the differences in bilirubin level in various experimental jaundice models. The Bil constituents in conjugated hyperbilirubinemic dog models were identified. Total Bil was measured using the Jendrassik and Grof method and direct spectrophotometry. Conjugated and unconjugated Bil were measured using the Weber-Schalm extraction method. Bilirubin covalently bound to albumin was measured indirectly from the total Bil and the non-CBBA fraction. The non-CBBA was estimated either as the sum of the CB and UCB concentrations determined by the W-S method or as the nonprecipitated fraction after deproteinization with ammonium sulfate-saturated ethanol when using DS. The TB, CB, UCB, and CBBA levels were compared in two hyperbilirubinemic dog models: chronic bile duct ligation (CBDL) and internal choledochocaval anastomosis (CDCA). The mean TB in internal CDCA (16.5 +/- 3.67 mg%) was significantly higher than in CBDL (3.4 +/- 1.75 mg%). Most of the serum Bil in these two models was conjugated, 13.4 +/- 2.24 and 3.2 +/- 1.7 mg%, respectively. No CBBA was found in the CBDL or in the partially obstructed internal-CDCA dogs. The TB in an external-CDCA model was essentially similar to the internal-CDCA model. The indirect Bil level in the external-CDCA model was six to seven times higher than the UCB level, and the CBBA level varied between 30 and 80% of the TB. Up to 50% CBBA was found also in patients with intra- or extrahepatic cholestasis. The findings indicate that, unlike the commonly assumed hypothesis, the serum TB level in the CDCA models, which was higher than in the CBCL one, is not due to high UCB levels. Rather, hyperbilirubinemia in the external-CDCA model is due to increased levels of CBBA or, perhaps, to variations in the amount and/or composition of CB entering the blood and cleared by the kidney.

Animals↗
PubMed1984

Tailor-made agarose-based reactive beads for hemoperfusion and plasma perfusion.

Composite beads of approximately 1 mm diameter, made of crosslinked agarose and containing Fuller's Earth or zirconium oxide powders, were prepared and used in extracorporeal systems for blood detoxification. The former was used for the removal of Paraquat, while the latter was used to remove inorganic phosphate from hyperphosphatemic animals with or without acute renal failure. The high surface area of the powder, combined with the low resistance to diffusion in the crosslinked agarose matrix, are highly advantageous. The crosslinking provides high mechanical strength, heat stability, prolonged shelf life, good blood flow characteristics, and prevents the release of fine particles into the blood. Crosslinked agarose beads of 1 mm diameter, containing chemically-bound heparin were also prepared, and used as a model for direct contact removal of LDL-cholesterol from the blood of familial hypercholesterolemic patients by hemoperfusion. The high capacity of these beads (over 5 mg LDL/mL beads) indicates that this clinical modality can replace the highly expensive plasmapheresis procedure presently used.

Animals↗
PubMed1984

Model for left ventricular contraction combining the force length velocity relationship with the time varying elastance theory.

A model for the contraction of the left ventricle (LV) is developed for a spheroidal geometry. The classical force-length-velocity relationship for a single muscle fiber is assumed. The linear maximum pressure volume relationship (maximum elastance), a measure of muscle contractility, is further extended into a time-varying function. This is achieved by utilizing a mechanical activation function, assumed as half a sinusoidal wave, to describe the time-dependent isometric stress for the activated cardiac muscle. This, in turn, results in the time-varying elastance function and represents the instantaneous activity of the muscle contractile proteins. The model is tested for a set of boundary conditions that determine preload, afterload, and the inherent properties of the muscle, i.e., the contractility. The computed results of the isovolumic contraction, auxotonic contraction, and isovolumic relaxation are in agreement with the expected behavior of the LV. The relations between the simulated variations on preload, afterload, and contractility, and the set of performance indexes of the LV, are presented and discussed.

Animals↗